Noninvasive Assessment of Cardiac Output: Accuracy and Precision of the Closed-Circuit Acetylene Rebreathing Technique for Cardiac Output Measurement

Abstract

Background Accurate assessment of cardiac output is critical to the diagnosis and management of various cardiac disease states; however, clinical standards of direct Fick and thermodilution are invasive. Noninvasive alternatives, such as closed-circuit acetylene (C₂H₂) rebreathing, warrant validation. Methods and Results We analyzed 10 clinical studies and all available cardiopulmonary stress tests performed in our laboratory that included a rebreathing method and direct Fick or thermodilution. Studies included healthy individuals and patients with clinical disease. Simultaneous cardiac output measurements were obtained under normovolemic, hypovolemic, and hypervolemic conditions, along with submaximal and maximal exercise. A total of 3198 measurements in 519 patients were analyzed (mean age, 59 years; 48% women). The C₂H₂ method was more precise than thermodilution in healthy individuals with half the typical error (TE; 0.34 L/min [r=0.92] and coefficient of variation, 7.2%) versus thermodilution (TE=0.67 [r=0.70] and coefficient of variation, 13.2%). In healthy individuals during supine rest and upright exercise, C₂H₂ correlated well with thermodilution (supine: r=0.84, TE=1.02; exercise: r=0.82, TE=2.36). In patients with clinical disease during supine rest, C₂H₂ correlated with thermodilution (r=0.85, TE=1.43). C₂H₂ was similar to thermodilution and nitrous oxide (N₂O) rebreathing technique compared with Fick in healthy adults (C₂H₂ rest: r=0.85, TE=0.84; C₂H₂ exercise: r=0.87, TE=2.39; thermodilution rest: r=0.72, TE=1.11; thermodilution exercise: r=0.73, TE=2.87; N₂O rest: r=0.82, TE=0.94; N₂O exercise: r=0.84, TE=2.18). The accuracy of the C₂H₂ and N₂O methods was excellent (r=0.99, TE=0.58). Conclusions The C₂H₂ rebreathing method is more precise than, and as accurate as, the thermodilution method in a variety of patients, with accuracy similar to an N₂O rebreathing method approved by the US Food and Drug Administration.

Keywords: Fick; acetylene; cardiac output; exercise; heart failure; noninvasive diagnostics; thermodilution.

Figure 1. Reliability of the acetylene (C₂H₂) rebreathing technique vs thermodilution method for assessment of Qc.

A, Reliability of C₂H₂ rebreathing for estimation of Qc; (B) Reliability of thermodilution for estimation of Qc. Qc indicates cardiac output.

Figure 2. Comparison of the accuracy of the acetylene (C₂H₂) rebreathing technique vs thermodilution method for the assessment of Qc in both healthy patients and those with clinical disease in the supine position at rest.

A, Accuracy of C₂H₂ rebreathing for estimation of Qc vs thermodilution in healthy patients; (B) Accuracy of C₂H₂ rebreathing for estimation of Qc vs thermodilution in a clinical population. Qc indicates cardiac output.

Figure 3. Comparison of the accuracy of the acetylene (C₂H₂) rebreathing technique and thermodilution method vs the direct Fick method for assessment of Qc in healthy patients in both the supine and upright positions at rest.

A, Accuracy of C₂H₂ rebreathing for estimation of Qc at rest; (B) Accuracy of thermodilution for estimation of Qc at rest. Qc indicates cardiac output.

Figure 4. Comparison of the accuracy of the acetylene (C₂H₂) rebreathing technique and thermodilution method vs the direct Fick method for assessment of Qc during upright exercise in healthy patients and those with clinical disease.

A, Accuracy of C₂H₂ rebreathing for estimation of Qc during exercise in healthy patients; (B) Accuracy of thermodilution for estimation of Qc during exercise in healthy patients; and (C) Accuracy of C₂H₂ rebreathing for estimation of Qc during exercise in clinical populations. Qc indicates cardiac output.

Figure 5. Comparison of the accuracy of the acetylene (C₂H₂) and nitrous oxide (N₂O) rebreathing techniques vs the direct Fick method for assessment of Qc in healthy patients during supine rest and upright exercise.

A, Accuracy of N₂O rebreathing for estimation of Qc during supine rest in healthy patients; (B) Accuracy of C₂H₂ rebreathing for estimation of Qc during supine rest in healthy patients; (C) Accuracy of N₂O rebreathing for estimation of Qc during exercise in healthy patients; and (D) Accuracy of C₂H₂ rebreathing for estimation of Qc during exercise in healthy patients. Qc indicates cardiac output.

Figure 6. Agreement between the (A) acetylene (C₂H₂) rebreathing and (B) nitrous oxide (N₂O) techniques for assessment of Qc.

Qc indicates cardiac output.